Ais Decoderby Neal
Arundale

Decodes
All 27 AIS message typesand 50 Binary Messages

The decoder will decode all the NMEA
VDM or VDO content
of
all 27 AIS message types including !AIVDM, !BSVDM and !ABVDM,
The decoder has been tested on an XP PC, and believe it should work on
all Windows PC's after 98. Other users report it works on Vista, Win 7,
Win 7-64 and Linux (Wine).

Lower Manhattan

Immingham UK Docks

IntroductionAis Decoder accepts
AIS data from an AIS Receiver, the Internet or a Local Network
connection, decodes the data and presents the decoded data in a form
suitable for display and analysis by mapping program (eg Google Earth
or Google Maps), or for analysis using Excel, or by a database (eg
MySql).

Main
Features

Input:

Serial or USB from AIS receiver, UDP or TCP from network,
Log File

Input Filter:

AIS Message type, DAC, FI, ID, MMSI

Output:

Display, File, FTP, UDP

Output Frequency

As received or at Scheduled intervals

Output Filter:

Lat/Lon or any other decoded field

Output Formats:

Text, CSV, HTML, XML, KML, KMZ, NMEA

Self-extracting
installation
file, including help

PDF Help Manual

AISAIS is primarily transmitted by ships for
collision avoidance. Ships over 200 tons must carry an AIS transponder
which both transmits and receives AIS data, enabling vessels to see the
speed and course (and other information) of within around 20 miles of one
another. It is also used by the Coast Guard and port authorities to
monitor vessel movements. Other uses are becoming more common, such as
Virtual Navigation Aids, weather and sea state reporting, control of
inland waterways such as canals and locks. Anyone with a suitable
receiver can receive this data.
AisDecoder Specification

There is a comprehensive Help file distributed
with the installation file
There is also a PDF extraction of this file here.

How AIS data is Encoded
AIS data is encapsulated within the Marine Standard NMEA protocol,
which defines Standands
for
communication
between
wired
electonic
ship
devices.
NMEA
defines
many
different
sentences,
two
of
which
are
normally
used
for
containing AIS data named !AIVDM
and !AIVDO. The content of both these sentences are identical (!AIVDM
is transmitted, !AIVDO is on board).
Transmission
For transmission the !AIVDM sentences are sent in individual packets
using a system called TDMA (Time Division Multiple Access). If two
transmitters sent data at exactly the same time, a receiver with range
of both transmitters would pick up garbled packets. TDMA organises AIS
transmitters in such a way that each AIS transmitter does not try and
transmit their own data packets at exactly the same time as another
transmitter within range. Each packet transmitted lasts for exactly
26.670 milliseconds and is transmitted on a VHF channel within the
marine radio band. The transmission of one packet is called a slot,
during which time exactly 168 bits of data can be sent. If a NMEA
sentence contains more than 168 bits, multiple slots are used, the
complete message being re-assembled by the AIS receiver. This splitting
of a AIS message into packets for transmission and the re-assembly by
the receiver is transparent to AisDecoder, however the control of
the slots uses specific AIS messages, which are decoded.NMEA Sentences
NMEA sentences are split into words separated by a comma, and have a
maximum length of 80 characters.
eg !AIVDM,1,1,,B,13PRrB0000OvbS@NhA9=oPbr0<0u,0*58
The first word !AIVDM is the name of the sentence, the last 2
characters are the checksum.The AIS Payload is
the 6th word - 13PRrB0000OvbS@NhA9=oPbr0<0uAs the maximum size of
the whole sentence is 80 characters, the payload can be no longer than
51 characters, and may be shorter.
Because the permitted characters in a NMEA sentence are a subset of the
8-bit ASCII character set, and the payload must be able to contain all
bits, each 8 bit character in the payload can only represent 6 bits of
data. Every 6 bits of AIS data has to be encoded into one 8 bit ASCII
character, prior to transmission. One NMEA AIS sentence can only
contain a maximum of 306 bits.
If an AIS message is longer than 306 bits, each AIS message must be
split into "multi-part" NMEA sentences. Words 2,3 and 4 are the no of
parts, this part and a part id. The 5th word is the AIS radio channel
which has been used for transmission. AIS Messages
AIS messages are contained entirely in the payload (6th) word of a NMEA
sentence or in the case of multi-part messages in mutiple payloads,
after re-assembly by the decoder. The messages, of which there are 27
basic types, are split into separate fields. Each field contains a
number of bits. These fields are also encoded as they could represent
numerical data (eg speed) or textual data (eg vessel's name) or require
conversion from a numeric code to a meaningful description (eg type of
vessel where 50 = Pilot Vessel). AisDecoder decodes around 1500
separate fields.
The first 3 fields of AIS messages are always Message type, Repeat
indicator and MMSI.Binary
AIS
MessagesAIS messages 6,8, 25 and 26 are called binary messages because
the content of the decoded message can in itself contain binary data.
In the case of the first two (6 and 8) these messages are further
subdivided to a further two or three sub levels of different types
messages.
The first sub level is by Designated Area Code (DAC) for example where DAC=1 is for
international messages.
The next sub level is by Function Identifier (FI) for example where 2 =
Vessel/Lock scheduling
The next sub level is by Identifier (ID) for example where 1 = Lockage
order